Why would my dog all of a sudden start peeing in my house?

It most likely is because of old age, and she knows she is not supposed to be doing that, but she can't help it. That's most likely why she does it when no one's around. And yes, when you first got her, it might have been because she was abused and was afraid to be alone. But I suggest taking her to the vet at all costs. She might have some sort of an infection, causing her to have accidents like that in the house. You might not like the answer they give you, but it's for the best. I wish you the best of luck with your dog, dear :)